1. Field of the Invention
The present invention relates to the preparation of ammonium rare earth double oxalates, and to the conversion of the double oxalates thus prepared into rare earth oxides.
This invention especially relates to the preparation of such double oxalates possessing a controlled and specific morphology and particle size.
2. Description of the Prior Art
The rare earth oxides find numerous applications in such fields as, in particular, ceramics and electronics, but, at the present time, an increasing demand exists for products having controlled particle size.
One of the convention processes for preparing rare earth oxides, and which is amply described in the literature, in particular in the Nouveau Traits De Chimie Minerale ("New Treatise on Inorganic Chemistry"), Volume VII, (1959), p. 1007 by Paul Pascal, entails calcining at temperatures ranging from 500.degree. to 900.degree. C. the rare earth oxalates prepared by precipitation, using oxalic acid, of rare earth salts in the form of an aqueous solution thereof. However, such a process produces only rare earth oxides possessing coarse particle sizes.
It is also known to this art, per JP 53/095,911-A (Chemical Abstracts, 90, 40940 w), to prepare finely divided rare earth oxides, and more especially finely divided yttrium oxide, by calcination of an ammonium/yttrium oxalate. This process comprises precipitating the yttrium in the form of its hydroxide from an aqueous solution of a yttrium salt. The aqueous solution of the yttrium salt is reacted with a basic aqueous solution such as ammonia solution, and the resulting hydroxide slurry is then treated with oxalic acid, and, finally, the resulting precipitate is separated, washed and calcined at a temperature of 750.degree. C. According to said JP 53/095911-A, a finely divided yttrium oxide is produced. Its particle diameter ranges from 0.9 to 4.5 .mu.m, the crystals having the shape of small plates with rounded edges.
However, control of the particle size, as well as of the particle size distribution, are relatively difficult, since these are greatly influenced by the conditions of actually carrying out such process.